It has 4 GB still, they just sectioned it into a 3.5 and .5 section. It is still able to use both for a combined 4 GB; there's just small bandwidth discrepancies between what they promised and actual performance.
Here's a relatively simple explanation of what is going on, along with some testing to show it doesn't affect performance to a noticeable degree: http://hexus.net/tech/news/graphics/79925-nvidia-explains-geforce-gtx-970s-memory-problems/